Liquid crystal apparatuses that utilize the electro-optical effect of nematic liquid crystals are being applied in a wider range than priorly. Their display systems include such as dynamic scattering types, quest-host types, twisted nematic types (TN types) and supertwisted nematic types (STN types), and the methods used to drive these display apparatuses include such as static drive systems, time sharing drive systems (dynamic drive systems), active matrix drive systems and 2-frequency drive systems.
The main characterizing features of TN types and STN types that make them superior to emission types of display apparatus such as LED, EL and CRT are such as the following.
(1) Miniaturization and thinning are possible.
(2) Drive voltage is low, and power consumption is very little.
(3) Phasing with LSI is good, and the drive circuit can be simplified.
(4) Because they are light receivers, they are easily seen in direct daylight, and do not cause eyestrain even under long periods of use.
Making the best use of these characterizing features, TN type display apparatuses are being widely applied in displays for such as watches, desk calculators, audio instruments, games, automobile dashboards, cameras, telephone instruments and various types of measuring instruments, and it is estimated that these fields of application will continue to broaden in the future.
Liquid display apparatuses have had their display capacities (number of scanning lines) expanded compared to prior art, together with the expansion in their fields of application. With TN type display apparatuses having time sharing drive systems, the number of scanning lines has a limit on the order of 200, and STN types and TN types with active matrix drive systems are being developed as display systems to replace them. At the present time STN systems are being applied to word processor and personal computer displays and TN types with active matrix drive systems are being applied to liquid crystal color televisions, with the prior main focus being as display apparatuses replacing CRT's.
The various properties indicated below are what are considered to be the indispensable requirements in TN type or STN type liquid crystal display apparatus for such applications, with the understanding that the properties of the liquid crystal materials used in these liquid crystal display apparatuses have various differences in such as their fields of application, display systems and drive methods.
(1) They must have thermal, optical, electrical and chemical stability, without coloration.
(2) They must have a practical temperature range as wide as possible in the vicinity of room temperature.
(3) The threshold value (V.sub.th) of the voltage-light transmission properties (V-I.sub.o properties) must be low, and the temperature dependence (.DELTA.T) must be little.
(4) The steepness (.beta.) of V-I.sub.o should be as sharp as possible.
(5) The visual angle dependence (.alpha.) of V-I.sub.o should be as small as possible.
(6) The electro-optical response speed should be fast.
Many liquid crystal compounds and liquid crystal analog compounds are known that satisfy (1) among these various properties, but at present none are known that satisfy the properties from (2) down with a single constituent liquid crystal compound. The actual situation is that liquid crystal compositions containing mixtures of plural nematic liquid crystal mixtures or their analogs are used to satisfy these properties.
After comparing the relation between the properties listed above against the display capacities (number of scanning lines) of the TN type liquid crystal display apparatuses with time sharing drives that are now most widely used, the indispensable requirements for high time sharing drives are these three.
(1) Steepness must be sharp.
(2) V.sub.th temperature dependence must be small.
(3) Visual angle dependence must be little. Considering LSI voltage capacity and power source voltage, there is also this important condition.
(4) The V.sub.th must be low.
These various conditions will be explained next. First, steepness (.beta.) will be represented as V.sub.th and V.sub.sat ##EQU1## Here V.sub.th and V.sub.sat are the voltages when the light transmissivity is respectively 10% and 90%.
Ideally .beta.=1, but it is actually on the order of .beta.=1.3 to 1.5, and it is difficult to develop liquid crystal compounds having a .beta. smaller than this value. In the case of the TN type, it is known that .beta. decreases as the value of K.sub.33 /K.sub.11 decreases, and it is reported that K.sub.33 /K.sub.11 is small in liquid crystal compounds having pyrimidine structures. Here, K.sub.11 and K.sub.33 are respectively the splay and bend elastic constants.
V.sub.th is expressed ideally for the TN type as follows. ##EQU2## Here d is the thickness of the liquid crystal layer, K.sub.11, K.sub.22 and K.sub.33 are respectively the splay, twist and bend constants of elasticity, and .epsilon..sub.o represents the permitivity of vacuum.
Consequently, to lower V.sub.th, the constant of elasticity should be small and a liquid crystal compound having a large .DELTA..epsilon. should be added. Also, the temperature dependence (.DELTA.T) of V.sub.th generally shows a tendency for the V.sub.th to lower along with rises in temperature, and has the particular characteristic of dropping sharply in the vicinity of the N-I point. This is because of the temperature dependence of the constant of elasticity.
Visual angle dependence (.alpha.) is a particular property of the TN type or the STN type, and its cause is said to lie in the pretilt arising between the liquid crystal and the surface given orientation treatment.
.alpha. can be improved by making the thickness of the liquid crystal layer thinner and making the An of the liquid crystal composition smaller.
In this regard, known liquid crystal compounds having a large .DELTA..epsilon. and a small An that were used for these purposes priorly are shown next. ##STR2##
These liquid crystal compounds have high N-I points and small .DELTA.n, but they have the defects of a small .DELTA..epsilon. and poor compatibility with other liquid crystal compounds.